This invention relates to a flow meter for measuring the mass rate of flow of a fluid. More particularly, this invention relates to a mass flow meter and flow measurement system for measuring the mass rate of flow of a fluid. Still more particularly, this invention relates to a mass flow meter utilizing pulse excitation and subsequent wave form analysis to calculate the Reynolds Number of the flow and to derive the mass flow rate from that calculation.
There are a significant number of applications which require a precise measurement of the mass flow rate of a fluid, particularly in the areas of process monitoring and control. Accordingly, a number of approaches have been developed for performing such a measurement and, on a historical basis, have largely involved mechanical or electromechanical techniques.
The measurement of fluid mass flow rate or velocity by utilizing thermal transfer techniques is well known. An example of one such device is the hot wire anemometer which was in general use and analyzed by King as early as 1914. In such devices generally, a heated element is placed in thermal contact with the flowing fluid and the rate of heat transfer from the heated element to the flowing fluid is measured. The rate of heat transfer is directly proportional to the flow Reynolds Number and generally follows the formula: EQU H=A+BR.sub.e.sup..alpha. P.sub.R.sup..beta. (1)
where:
H is the thermal transfer coefficient; PA1 A & B are constants; PA1 R.sub.e is the Reynolds Number; PA1 .alpha. is the Reynolds power; PA1 P.sub.R is the Prandtl Number; and PA1 .beta. is the Prandtl power. PA1 E is a constant; PA1 W is the mass flow rate; PA1 d is the pipe inside diameter; and PA1 .mu. is the fluid viscosity. From this equation it can be readily seen that the relationship between W, the mass flow rate, and H, the coefficieint of the heat transfer can be interpreted by common instrumentation directly into terms of mass flow rates. There are several commercial applications of this technology.
The Reynolds Number can be expressed by the equation: EQU R.sub.e =EW/d.mu. (2)
where:
A basic difficulty with current devices applying thermal transfer techniques is that, in order to achieve a practical sensitivity, the temperature difference between the heated element and the flowing fluid must be on the order of 10.degree. C. to 30.degree. C. Unfortunately, there are a number of fluids which cannot tolerate this heating without undesirable chemical changes. In addition, there are also fluids which have the tendency to deposit upon or coat any surface which is but a few degrees different from, either above or below, the process temperature. Examples of such fluids are blood, latex, epoxies, starch, and fine clay suspensions.
It is thus a basic aim of this invention to provide a flow meter utilizing thermal techniques which avoids the need for significant temperature differences between the heated element and flowing fluid while continuing to achieve a practical sensitivity for commercial application, such as on the order of 1 to 2 percent or better.
In the semiconductor field, thermal pulses of very short duration have been used to measure the properties of insulating and conducting crystals. The heat pulse experiments to measure those properties utilized as basic elements a small heater or thermal transducer to produce an excitation of a known pulse width and a thermal receiver whose response is proportional to the incident thermal flux. An example of this application in the semiconductor field utilizes an evaporated thin film heater and detector circuits on opposite polished faces of a crystal.
However, such thermal pulse techniques have not apparently been applied to the manufacture of commercial flow meters. Thus, an additional general object of this invention is to provide a flow meter for measuring the mass rate of flow of a fluid utilizing pulse excitation techniques to permit a wave analysis of the response of the flow transducer to the excitation pulse in order to calculate the Reynolds Number and derive the mass flow rate from that calculation.
It is another overall purpose of this invention to retain all of the advantages of the thermal transfer technology while eliminating the problems of excessive flow element temperatures. In addition, the flow meter according to the invention will provide the advantage of greatly simplified transducers and the ability to use common electronics for multiple flow transducers as will be described in detail in the specification.